Nanoparticles switch phases in flight

Wednesday, 27 February, 2019

Researchers from the Hebrew University of Jerusalem and Cornell University have made a breakthrough nanoscience discovery, revealing that a ‘magic-size nanocluster’ is the missing link that bridges the divide between how matter rearranges itself in small-scale molecular isomerisation and in large, solid bulk matter phase transitions. Their work has been published in the journal Science.

Isomerisation — the transformation of a molecule into another molecule with the same atoms, just in a different arrangement — is common in nature. Often it’s sparked by the addition of energy, as when light causes a molecule in the retina to switch, enabling us to see; or how olive oil, when heated too high, isomerises into the unhealthy form known as a trans-fat. Bulk materials such as graphite can also change phases — into diamonds, for example — but they require a lot more energy and the change occurs more gradually, with the change spreading slowly across the molecule.

But can isomerisation occur in inorganic materials? The answer to this question came about quite by accident, when Cornell University’s Professor Richard Robinson was on sabbatical at the Hebrew University and he asked a graduate student from back in New York to mail him some nanoparticles of a specific size.

“When they got to me, I measured them with the spectrometer and I said, ‘Wait, you sent me the smaller particles instead of the bigger ones,’” Prof Robinson said. “And he said, ‘No, I sent you the bigger ones.’ We realised they must have changed while they were in flight. And that unleashed a cascade of questions and experiments that led us to this new finding.”

Prof Robinson and Hebrew University colleague Professor Uri Banin deduced that the particles had transformed during their trip from Ithaca to Jerusalem. This realisation led to the discovery of inorganic isomerisation, in which inorganic materials are able to switch between discrete states almost instantaneously — faster than the speed of sound. It also bridges the gap between what’s known about phase changes in organic molecules and in bulk materials.

“We found that if you shrink inorganic material small enough, it can easily jump back and forth between two discrete phases, initiated by small amounts of alcohol or moisture on the surface,” said Prof Robinson. “On the flight there must have been moisture in the cargo bin, and the samples switched their phase.”

“We bridged the two worlds between big materials that change more slowly and small, organic materials that can flip back and forth coherently, between two states. It’s surprising that we saw an instantaneous transformation from one state to another in an inorganic material, and it’s surprising that it is initiated with a simple surface reaction.”

In the past, larger nanoparticles were found to change phases in a way that was closer to how bulk materials change than to molecules. But when the Cornell team looked at even smaller clusters of atoms at the Cornell High Energy Synchrotron Source (CHESS), where they performed total-scattering experiments in which they examined all the X-ray scatterings of the cluster, they observed the quick change between discrete states for the first time.

“We now finally see that there’s a new regime where you can coherently flip from one state to another instantaneously,” said Cornell University Professor Tobias Hanrath. “If you make them small enough, the inorganic materials can flip back and forth very easily. It’s a revelation.”

The researchers were also aided by a new technique they developed to create ‘magic-sized nanoclusters’ — so-called because they have the ‘perfect’ number of atoms and no more individual atoms can be added, making them extremely stable. These clusters contain only 57 atoms — making them bigger than your typical molecules but still much smaller than bulk materials, like graphite or diamonds.

The team revealed that the transition in these clusters — as they changed from one structure (or phase) to another — took place in a single step, as is the case for isomerisation in small molecules. In this way, the team found the missing link between bulk phase transitions and molecular isomerisation.

“We were able to come up with a very pure magic-sized cluster,” Prof Robinson said. “Because of that, when it reacts with the alcohol or water you see a very pure transformation” — from one discrete state to another.

Though further research is needed, possible future applications include using these particles as switches in computing or as sensors. The discovery could also have uses relating to quantum computing or as a seed for the generation of larger nanoparticles.

“Nanoclusters are chemicals that can be used to create other larger materials,” Prof Banin said. “Being able to manipulate their precise changeover from one state to another could have many significant applications down the road.”

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